Electrolyzation (hereinafter referred to as “electrolysis”) of water is performed for producing electrolytic water employed in the fields of medical care and food or for cleaning electronic components, or for wastewater treatment. For example, electrolysis of water is known as a method for producing water in which ozone is dissolved, i.e., the so-called ozone water.
The ozone water is a kind of functional water, has extremely strong oxidizing power, and is utilized in various fields of medical care, cleaning of electronic devices etc. as sterilizing water or cleaning water through the oxidizing power. In order to form ozone water by dissolving ozone gas in water, an apparatus for generating impurity-free ozone gas and dissolving the same in water is necessary, and the apparatus is increased in size and complicated, while it is difficult to obtain high-purity ozone water. According to a method for forming ozone water by electrolyzing pure water, however, the apparatus can be easily downsized, and high-purity ozone water can be easily obtained.
Lead dioxide, platinum or the like excellently functioning as a catalyst is known as the material for an electrode employed for this electrolysis. However, this material has a problem of elution of the electrode, and an apparatus for removing eluted metallic impurities must be provided when high-purity ozone water is required, to complicate the apparatus.
Therefore, a conductive diamond supplied with conductivity through addition of a dopant such as boron has recently been attracted attention as an electrode material substituting for the aforementioned material. The conductive diamond is chemically extremely stable, not eluted in electrolysis and has a wide potential window, whereby ozone can be electrolytically generated from pure water having high electrical resistance with an electrode (hereinafter referred to as “diamond electrode”) employing this conductive diamond.
As this diamond electrode, Japanese Patent Laying-Open No. 2005-336607 (Patent Document 1), for example, discloses an electrode obtained by perforating and meshing a self-supporting membrane of diamond prepared by chemical vapor deposition (CVD) in order to increase the surface area of the electrode and improve electrolytic efficiency. However, a long synthesis time is required for preparing this diamond self-supporting membrane to result in a high cost, while the diamond self-supporting membrane is easily warped and easily forms a clearance between the same and an ion-exchange membrane. Further, the step of meshing the membrane with a laser also results in a high cost.
Therefore, a diamond electrode obtained by forming a membrane of conductive diamond on a meshed or porous substrate consisting of a valve metal such as titanium or niobium by CVD is proposed, and disclosed in Japanese Patent Laying-Open No. 9-268395 (Patent Document 2), Japanese Patent Laying-Open No. 2001-192874 (Patent Document 3) or the like, for example. However, the thermal expansion coefficient of the substrate consisting of titanium or niobium is remarkably different from that of diamond, whereby the electrode easily cause remarkable residual stress resulting from the difference in thermal expansion coefficient between the same and diamond when in use. Consequently, the conductive diamond and the substrate are easily separated from each other, to disadvantageously reduce the life of the electrode.
As a substrate for forming a diamond electrode by covering conductive diamond by CVD, a conductive silicon substrate is also known. The difference in thermal expansion coefficient between the conductive silicon substrate and diamond is relatively small, whereby the conductive silicon substrate has a small problem of separation resulting from residual stress. However, it is difficult to form a mesh structure with conductive silicon. In other words, while a substrate of a mesh structure can be easily produced by a method laterally pulling a flat plate provided with a large number of small pores in the case of titanium or niobium, this method cannot be applied to conductive silicon, and only a flat conductive silicon substrate has been present in general. Therefore, no porous diamond electrode employing a conductive silicon substrate has been obtained.    Patent Document 1: Japanese Patent Laying-Open No. 2005-336607    Patent Document 2: Japanese Patent Laying-Open No. 9-268395    Patent Document 3 Japanese Patent Laying-Open No. 2001-192874